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ADS1118: Does the temperature sensor measure the ambient temperature or device temperature?

Betty Guo
Expert 7300 points
Part Number: ADS1118

Hi team,

customer use ADS1118 for thermocouple measurement and use temperature integrated in device for compensation of thermocouples. 

here is a question is that does is measure ambient temperature or its self temperature?

if it measure itself temperature, it will have error because the device will heats. In customer project, when the start, the IC temperature is 28℃, after it work some minutes, the temperature will up to 34℃.

could you give some information about how to use temperature sensor to compensate the thermocouple  accurately?

in the datasheet, it show that the sensor can be used for system-level temperature monitoring or cold-junction compensation for thermocouple. what's the system -level? and how to compensate without being influenced by itself?

thank you

  • 0
    TI__Guru* 95075 points
    Betty,


    The device measures the from the die. There is temperature sensor based on a diode in the die. Because the board and the device will likely have little thermal impedance, the board and device temperature should be the same.

    Normally, you would design the ADS1118 board so that the device sits on a cold junction that is large and can accurately matche the ambient temperature (or at least the cold junction temperature). The device cannot measure anything it doesn't have physical contact to.

    I would note that the device itself should not add much heat to the temperature at all. If you look at the device specifications, the device uses 150uA at 3.3V. That is the equivalent of 0.0005W. Then if you look at the junction-to-board thermal resistance, this is 108.4°C/W for the DGS package. With the power dissipation, you would expect that the device temperature is only 0.054°C rise above the board temperature. If you use single-shot conversion mode, you could reduce the temperature contribution of the ADS1118 even more. Regardless the self-heating of the device is negligible.

    If there are other devices that dissipate heat, they must be thermally isolated. They must be either be moved further away, or placed on another board. Again you want the ADS1118 on a board that matches the cold junction temperature. If you go to one of the TI designs, there is a discussion on the PCB layout of this type of board using the ADS1118:

    www.ti.com/.../slau509.pdf

    It isolates the ADS1118 away from the processor, and uses a ground fill cut to further isolate the device. Regardless, the document describes this type of circuit in detail and is a good reference for the design.


    Joseph Wu